Christchurch's former central library is expected to be demolished by mid-October.

The Gloucester St building has been closed since the February 2011 earthquake and the site has been earmarked for the city's new convention centre.

A Canterbury Earthquake Recovery Authority (Cera) spokeswoman said the library's insurance company was removing structural samples from the building to test for strength and condition. The site was expected to be handed over to Cera in two or three weeks, and demolition work would take about five weeks to complete, she said.

Construction of a new library fronting Cathedral Square is scheduled to begin later this year.

The $85 million facility will be the largest public library in the South Island and will be able to cater for 2000 visitors a day when it opens in 2017.